This invention relates generally to wellheads and deals more particularly with a mechanism for holding down a tubing hanger in a unitized wellhead.
In a typical wellhead structure, one or more concentric casing strings are suspended from casing hangers which are supported within the bore of a single unitized head member. After the casing hangers have been properly situated, a hanger bowl is lowered into the wellhead to provide support for a tubing hanger which is usually considerably smaller in cross section than the bore of the wellhead, thus necessitating installation of the hanger bowl. The tubing hanger receives one or more tubing strings which extend down into the well within the innermost casing string.
The casing hangers are normally held down by a plurality of simple holddown screws which also act to energize pack-offs or other seal devices associated with the casing hangers. The holddown screws are threaded into the wellhead and against camming surfaces of the casing hangers or pack-off devices in order to urge these components downwardly. Holddown screws of this type may be used in a similar manner to hold down the tubing hanger of a more conventional type of wellhead, wherein the tubing hanger is mounted directly on a shoulder of a tubing head without an intermediate hanger bowl interposed between the hanger and the tubing head. However, in a unitized wellhead, the presence of the hanger bowl between the wellhead and the tubing hanger makes it impractical to employ simple screws to hold down the tubing hanger and activate its associated seal. In this instance, each screw must be run in a considerable distance through both the wellhead and the hanger bowl, requiring a rather long screw which is subjected to considerable axial stresses that tend to bend or otherwise deform and damage the screw. In addition, a difficult alignment problem is encountered because each passage of the hanger bowl must be precisely aligned with the corresponding passage of the wellhead in order for a single long screw to be run in successfully.
U.S. Pat. No. 3,489,439 to Word discloses one type of mechanism that is intended for use to hold down the tubing hanger of a wellhead. Although this arrangement avoids the problems associated with the use of simple holddown screws, it is less than satisfactory in a number of other respects, most notably in its cost and complexity. For example, the use of a camming ring or gland having a frusto-conical shape unduly complicates the structure and adds to the difficulties involved in fabricating and assembling the wellhead. Moreoever, three separate components must interact with one another in camming fashion which requires accurate dimensioning and precise machining of the parts if they are to cooperate with one another properly. The complicated arrangement of the components and their interaction with one another also detracts significantly from the reliability of the mechanism, particularly after extensive use and the accompanying wear to which the parts are subjected. Additional drawbacks include the lack of any positive means for retaining the inner pins properly in place during assembly of the wellhead and the lack of a positive release of the pins from the groove of the tubing hanger when the screws are backed off to permit lifting of the tubing strings.